MINI-FLUIDICS CASSETTE FOR COLORIMETRIC NUTRIENT ANALYSIS AND A METHOD OF USING SAME

Abstract

A mini-fluidics cassette, for detection of at least one analyte in a sample, comprising, at least one sample inlet port, at least one reagent inlet port, at least one outlet port, at least one channel extending between said at least one sample inlet port and said at least one outlet port, at least one insertion port for a fiber optic cable light source, at least one insertion port for a fiber optic cable spectrophotometer distant said at least one insertion port for a fiber optic cable light source, wherein said at least one insertion port for a fiber optic cable light source and said at least one insertion port for a fiber optic cable spectrophotometer forms part of the at least one channel, and is proximate said at least one outlet port and forms at least one reading cell/path length for light from said fiber optic cable light source to said fiber optic cable spectrophotometer port.

Claims

1. A mini-fluidics cassette, for detection of at least one analyte in a sample, comprising: i) At least one sample inlet port; ii) At least one reagent inlet port; iii) At least one outlet port; iv) At least one channel extending between said at least one sample inlet port and said at least one outlet port; v) At least one insertion port for a fiber optic cable light source; vi) At least one insertion port for a fiber optic cable spectrophotometer distant said at least one insertion port for a fiber optic cable light source; wherein said at least one insertion port for a fiber optic cable light source and said at least one insertion port for a fiber optic cable spectrophotometer forms part of the at least one channel, and is proximate said at least one outlet port and forms at least one reading cell/path length for light from said fiber optic cable light source to said fiber optic cable spectrophotometer port; wherein when said analyte is a nitrate, said at least one channel further comprises an inlaid cadmium channel portion between said at least one reagent inlet port and said at least second reagent inlet port.

2. The mini-fluidics cassette of claim 1 further comprising at least one second reagent inlet port; said second reagent inlet port distant said at least one reagent inlet port.

3. The mini-fluidics cassette of claim 1 wherein said at least one channel is serpentine.

4. The mini-fluidics cassette of claim 1 said at least one analyte is selected from the group consisting of nitrate, nitrite, ammonium, phosphate, silica, bromide and sulfate.

5. The mini-fluidics cassette of claim 4 wherein said at least one analyte comprises at least two analytes.

6. The mini-fluidics cassette of claim 4 wherein said at least one analyte is in the form of a solution.

7. The mini-fluidics cassette of claim 1 wherein said reading cell/path runs substantially the length of said cassette.

8. The mini-fluidics cassette of claim 1 wherein said at least one outlet port is a waste port.

9. The mini-fluidics cassette of claim 1 wherein said cassette is manufactured of a material resistant to said at least one analyte.

10. The mini-fluidics cassette of claim 9 wherein said material is selected from the group consisting of plastic, high-density polyethylene, high-density poly propylene and combinations thereof.

11. The mini-fluidics cassette of claim 9 wherein said cassette is manufactured by 3D printing or injection moulding.

12. A system for analysis of at least one analyte in a liquid, said system comprising: i) At least one mini-fluidics cassette comprising: At least one sample inlet port; At least one reagent inlet port; At least one outlet port; At least one channel extending between said at least one sample inlet port and said at least one outlet port; At least one insertion port for a fiber optic cable light source; and At least one insertion port for a fiber optic cable spectrophotometer distant said at least one insertion port for a fiber optic cable light source; wherein said at least one insertion port for a fiber optic cable light source and said at least one insertion port for a fiber optic cable spectrophotometer forms part of the at least one channel, and is proximate said at least one outlet port and forms at least one reading cell/path length for light from said fiber optic cable light source to said fiber optic cable spectrophotometer port; wherein when said analyte is a nitrate, said at least one channel further comprises an inlaid cadmium channel portion between said at least one reagent inlet port and said at least second reagent inlet port; ii) at least one pump for pumping said liquid and at least one reagent through said cassette; iii) at least one light source; and iv) at least one spectrophotometer.

13. The system of claim 12 wherein the at least one mini-fluidics cassette comprises a plurality of mini-fluidics cassettes; i) wherein each of said plurality of mini-fluidics cassettes is adapted for analysis of a discrete analyte allowing for the detection and quantification of said discrete analyte in said liquid; iv) .

14. A method of analyzing at least one analyte in at least one sample, using at least one mini-fluidics cassette, said method comprising the steps of: 1) introducing the at least one sample containing the at least one analyte to said at least one mini-fluidics cassette; 2) introducing at least one reagent to said at least one mini-fluidics cassette; 3) contacting the at least one analyte with the at least one reagent; 4) conveying said contacted at least one analyte and at least one reagent through said mini-fluidics cassette a predetermined time allowing for mixing of said analyte with said reagent; 5) illuminating said contacted at least one analyte and at least one reagent with said fibre optic cable light source; and 6) measuring absorbance of said light by said contacted at least one analyte and at least one reagent with said fibre optic cable spectrophotometer.

15. The method of claim 14 wherein the at least one sample has at least one analyte selected from the group consisting of nitrate, nitrite, ammonium, phosphate, silica, bromide and sulfate.

16. The method of claim 14 wherein the at least one sample is in the form of a solution.

17. The method of claim 16 wherein the at least one sample is in the form of an aqueous solution.

18. The method of claim 14, wherein the contacted at least one analyte and at least one reagent is conveyed through said cassette using a pump.

19. The method of claim 18 wherein said pump is a peristaltic pump.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0038] FIG. 1 is a cross sectional view of a mini-fluidic cassette specific to nitrate analysis

[0039] FIG. 2 is a cross sectional view of a mini-fluidic cassette specific to ammonium analysis

[0040] FIG. 3 is a cross sectional view of a mini-fluidic cassette specific to phosphate analysis

[0041] FIG. 4 is a cross sectional view of a mini-fluidic cassette specific to sulfate analysis

[0042] FIG. 5 is a cross sectional view of the reading cell/path-length of a mini-fluidic cassette

[0043] FIG. 6 is a perspective view of a system comprising a plurality of mini-fluidic cassettes analyzing a sample for a plurality of analytes

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Referring to FIG. 1, there is provided a cross sectional view of a cassette specific to nitrate analysis of a liquid sample, providing for automated mixing of sample and reagents, and in-cassette measurement of light absorbance. There is provided a sample inlet port 10, a first reagent (buffer) inlet port 20, a second reagent inlet port 30, a first confluence zone 40 for sample and buffer, a channel for inlaid cadmium wire 50, a second confluence zone 60 for second reagent and combined sample/buffer, a reading cell/path-length for light from fiber-optic source to reading fiber 70, an insertion port for fiber-optic cable from light source 80, an insertion port for fiber-optic cable connected to spectrophotometer 90 (See FIG. 5), and an outlet port (to waste) 100. The cassette has a channel 110 to allow the sample from sample inlet port 10, first reagent (buffer) from first reagent inlet port 20 and second reagent from second reagent inlet port 30 to mix and travel through channel 110, through reading cell/path length 70 allowing a reading to be taken and sent to the spectrophotometer and out waste port 100. In this embodiment, the channel 110 is serpentine other than the inlaid cadmium wire channel 50 and reading cell/path length 70. In this manner, mixing of the sample with the reagents is maximized and sufficient transit time through the cassette is provided for chemical reaction and colour development.

[0045] In this embodiment, the cadmium wire channel 50 and the reading cell/path length 70 runs the substantial length of the cassette allowing for colour development.

[0046] Referring to FIG. 2, there is provided a cross sectional view of a cassette specific to ammonium with similar elements as the cassette of FIG. 1 except for the lack of a cadmium wire inlay section along the channel. In all respects the cassette of FIG. 2 functions in a similar manner as that described in FIG. 1, except it is specific to ammonium and the sample inlet port 10 and first reagent inlet port 20 meet at a first confluence zone 40 and said mixed sample then travels past a second reagent inlet port 30 to a second confluence zone 60 prior to entering the serpentine channel 110.

[0047] Referring now to FIG. 3, there is provided a cross sectional view of a cassette specific to phosphate with similar elements as the cassette of FIG. 1 except for the lack of a cadmium wire inlay section along the channel and there is a single reagent inlet port 20. In all respects the cassette of FIG. 3 functions in a similar manner as that described in FIG. 1, except it is specific to phosphate.

[0048] Referring now to FIG. 4, there is provided a cross sectional view of a cassette specific to sulfate with similar elements as the cassette of FIG. 3 except for the lack of a cadmium wire inlay section along the channel and the channel extends outside of the cassette perimeter for in-line insertion of a cartridge 120 containing an exchange resin for removal of potentially-interfering anions. In all respects the cassette of FIG. 4 functions in a similar manner as that described in FIG. 3, except it is specific to sulfate.

[0049] Referring now to FIG. 6, there is provided a perspective view of a system comprising a plurality of mini-fluidic cassettes analyzing a sample for a plurality of analytes. In this depiction, three mini fluidics cassettes 130, one for phosphate analyte 131, one for ammonium analyte 132 and one for nitrate analyte 133, each receive a portion of sample volume 140 via a multi channel peristaltic pump 150, each are connected to a fiber optic light source 160 and multi-channel fiber optic spectrophotometer 170 and each are connected to a waste disposal unit 180. The nitrate cassette receives reagents D 220 and E 230, ammonium cassette receives reagents B 200 and C 210, and the phosphate cassette receives reagent A 190, each reagent being specific to the desired analyte to be analyzed.

[0050] In standard colorimetric analyses, specific analytes in a water sample react with chemical reagents to form coloured products (dyes). The amount of dye produced is proportional to the amount of the analyte, and may be quantified using spectrophotometry.

[0051] The cassettes described herein automate the mixing of water samples and chemical reagents used in routine colorimetric nutrient analyses, and the cassettes each have an embedded reading cell for measurement of light absorbance by the produced dyes. The reading cell portion of the cassette has ports for connection of a fiber optic light source and a reading fiber that connects to a spectrophotometer.

[0052] Each cassette operates as a flow-through cell. The cassette contains inlet ports and a serpentine channel (having a diameter in the range of <1-5 mm) that runs throughout the cassette. The diameter of each port determines the cross sectional area of the channel, and therefore the volume of sample or reagent passing through the channel in a given interval. The diameters, thus, control mixing ratios to optimize dye production. The ports introduce water samples and reagents. The sample and reagents are combined and mixed within the serpentine channels. The mixing enhances chemical reaction between analytes in the water sample and the reagents. The serpentine channel increases the total path-length of the combined sample/reagent stream increasing time for color development. The sample flows through the reading cell. As best seen in FIG. 5, a beam of focused light passes through the glass window 240 at one end of the cell, through the sample (in this embodiment a 4 cm path-length) and through a glass window 241 at the opposite end of the cell. The light source is an optical fiber inserted into a threaded port 81 at one end of the cell. A second optical fiber inserted into a threaded port 91 on the opposite end of the reading cell transmits light to spectrophotometer. The absorption of light as it passes through the sample is recorded and may be used to quantify the dye produced within the sample, which is proportional to the analyte concentration. The sample then passes from the reading cell to waste through an exit port 100.

[0053] In a preferred embodiment each cassette is manufactured of plastic (e.g. HDPP or HDPE). Manufacturer may use 3D printing or injection molding or any other suitable process. The cassette may be produced as a solid piece or in mirrored halves to allow inlaying of a reactant material (e.g. cadmium wire for nitrate reduction) within the fluid channel. If manufactured in mirrored halves, the halves may be glued together with an adhesive appropriate for the material. When manufactured, the channel that comprises the reading cell is continuous with the fiber optic insertion ports. A tap is used to thread the ports for insertion of optical fiber collimating lenses. Circular glass cover slips (5-8 mm diameter) are then inserted into the threaded port of each reading cell and attached to the shoulder created by the narrowing of the channel to the reading cell. This creates a window on either end of the reading cell that physically separates the reading cell from the fiber optic ports (as best seen in FIG. 5).

[0054] In use, a multichannel peristaltic pump may be used to introduce the water sample and reagents to the cassette. Use of a multichannel pump and a multichannel fiber optic spectrophotometer allows the use of several cassettes in parallel for simultaneous measurement of a variety of analytes (e.g. nitrate, ammonium, phosphate, silica, sulfate, bromide), as best seen in FIG. 6.

[0055] The following are examples of the mini-fluidics cassettes and their use in analysis

EXAMPLE 1

Nitrate Cassette

[0056] The nitrate cassette includes: a sample inlet port, 0.1 cm diameter, buffer inlet port, 0.18 cm diameter, colour reagent inlet port, 0.09 cm diameter, threaded port for fiber optic cable insertion, connection to spectrophotometer, threaded port for fiber optic cable insertion, light source, waste outlet port, 0.25 cm diameter, and channel for inlay of cadmium wire.

[0057] The cassette was produced by 3D printing in high density polyethylene. The cassette was printed in two mirrored halves and sealed using HPDE adhesive (Reltek's BONDiT A-43) to attach the two halves of each cassette, although other methods of bonding (e.g. chemical etching with use of adhesives, melting) may be used. An 8 mm diameter glass cover-slip was inserted into each fiber optic port.

[0058] A multichannel peristaltic pump is used to introduce a water sample and buffer (ammonium chloride-EDTA solution) through sample inlet port and buffer inlet port, respectively. The water sample and the buffer are mixed in a 1:3 ratio and pass through the serpentine channel to facilitate mixing. The mixed sample then passes through a channel containing a fine copperized cadmium wire to reduce nitrate to nitrite. Colour reagent (sulfanilamide/N-1-naphthylethylenediamine dihydrochloride solution) is added to the sample through colour reagent port, using another channel of the peristaltic pump. The sample passes through serpentine channels to mix the combined sample and provide time for a dye-producing chemical reaction between nitrite and sulfanilamide/N-1-naphthylethylenediamine dihydrochloride. The nitrite produced by reduction on the cadmium wire is diazotized with sulphanilamide, and coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a red azo dye. The sample then enters a reading cell with a 4 cm path-length. A fiber optic light source is inserted into a threaded port. Light passes through a glass window sealing the end of the reading cell near the light inlet port, through the sample in the reading cell, and through a second glass window sealing the far end of the reading cell. An optical fiber is inserted into a threaded port, conveying light to a multichannel spectrophotometer. The spectrophotometer quantifies light absorbed at 543 nm. The absorption of light at 543 nm is proportional to the amount of dye, which is, in turn, proportional to the original concentration of nitrate (+nitrite).

[0059] Peristaltic pump speed may be controlled to increase or decrease reaction time, depending on sensitivity required versus speed of analysis. The minimum sample volume required for analysis is ˜650 μL. The minimum buffer volume required is ˜2 mL, and the minimum required volume of sulfanilamide/N-1-naphthylethylenediamine dihydrochloride solution is ˜600 μL, creating a total waste per sample of ˜3.3 mL.

[0060] Procedure:

[0061] Step 1—Copperizing cadmium wire: hydrochloric acid is run through cassette for a minimum of 5 minutes. This is then followed by running copper sulfate solution through the cassette for 10 minutes, followed by EDTA buffer for 5 minutes.

[0062] Step 2—Conditioning wire: run 100 μM potassium nitrate solution through the cassette for 5 minutes, followed by EDTA buffer for 5 minutes.

[0063] Step 3—Connect tubing: connect peristaltic tubing to ports (2.10 mm i.d. to ports 10 and 30, 2.79 mm i.d. to port 20).

[0064] Step 4—Run samples: pump water sample into port 10, EDTA buffer into port 20 and sulphanilamide/N-1-naphthylethylenediamine dihydrochloride solution into port 30.

[0065] Step 5—Record absorbance: once waste begins to exit through outlet port, stop pump and allow 5-10 minutes for colour development and then record absorbance from fiber optic spectrophotometer.

[0066] Step 6—Flush cassette: flush between samples with EDTA buffer (into port 20) and distilled water into ports 10 and 30.

[0067] Step 7—Storage: after analyzing samples, do a final flush of cassette with EDTA buffer and store cassette filled with buffer.

[0068] Reagents:

[0069] Sulphanilamide/N-1-naphthylethylenediamine dihydrochloride solution: To approximately 800 mL of reagent distilled water (deionized water may also be used), add, while stirring, 100 mL concentrated, phosphoric acid (CASRN 7664-38-2), 40 g sulfanilamide (CASRN 63-74-1) and 2 g N-1-naphthylethylenediamine dihydrochloride (CASRN 1465-25-4) reagent grade. Stir until dissolved and dilute to 1 L. Store in brown bottle and keep in the dark when not in use. This solution is stable for several months.

[0070] EDTA buffer solution: Dissolve 85 g of reagent grade ammonium chloride (CASRN 12125-02-9) and 0.1 g of disodium ethylenediamine tetracetate (CASRN 6381-92-6) in 900 mL of distilled water. Adjust the pH to 9.1 for preserved or 8.5 for non-preserved samples with concentrated ammonium hydroxide (CASRN 1336-21-6) and dilute to 1 L. Add 0.5 mL Brij-35 (CASRN 9002-92-0).

[0071] Comparison with APHA (1997) Standard Method 4500-NO.sub.3.sup.−E—cadmium reduction column: Nitrate was analyzed using a cassette as described above, and also using APHA method 4500-NO.sub.3.sup.−E. For each method, a standard curve was generated using working standards containing 0, 5, 10, 15, 20, and 25 μM NO.sub.3.sup.−. Samples were collected from 5 different water sources. For quality control, each sample was analyzed with and without a nitrate spike (10 μM) and a known standard (5 μM) was analyzed after every 5 samples. The detection limit for each method was calculated as 3 times the minimum nitrate concentration necessary to generate a signal above background (0 μM nitrate). The two methods had comparable limits of detection for nitrate (3.0 μM for cassette method, 3.7 μM for method 4500-NO.sub.3.sup.−E). The cassette had an average percent recovery in the five spiked samples of 100%, while method 4500-NO.sub.3.sup.−E had an average percent recoveries of 197%, suggesting the cadmium reduction column was inefficient in reducing nitrate to nitrite at low concentrations. Calculated concentrations in control samples were similar between methods (4.27 μM for cassette method, 5.33 μM for method 4500-NO.sub.3.sup.−E. Generally, the cassette yielded similar results to the traditional wet chemistry methods while resulting in much less waste (˜2 mL per sample versus ˜20 mL per sample).

EXAMPLE 2

Ammonium Cassette

[0072] Similar to Example 1, a multichannel peristaltic pump is used to introduce a water sample, phenol/nitroprusside solution, and oxidizing solution through 10, 20, and 30 (FIG. 2), respectively. Phenol reacts with ammonium in the water sample and hypochlorite in the oxidizing solution. The reaction, catalyzed by nitroprusside, produces indophenol, an intensely blue compound. The combined sample enters reading cell 70 with a 4 cm path-length. Light absorbance at 640 nm is measured using a fiber optic light source and spectrophotometer. The absorption of light at this wavelength is proportional to the amount of indophenol, which is, in turn, proportional to the original concentration of ammonium in the sample.

[0073] Peristaltic pump speed can be controlled to increase or decrease reaction time, depending on sensitivity required versus speed of analysis. The minimum volume of sample, phenol/nitroprusside solution, and oxidizing solution required for analysis is ˜1.2 mL, creating a total waste per sample of <3.6 mL.

[0074] Procedure:

[0075] Step 1—Connect tubing: connect peristaltic tubing to ports (2.79 mm i.d. to ports 10, 20, and 30).

[0076] Step 2—Run samples: pump water sample into port 10, phenol/nitroprusside solution into port 20, oxidizing solution into port 30.

[0077] Step 3—Record absorbance: once waste begins to exit through outlet port, stop pump and allow 5-10 minutes for colour development and then record absorbance from fiber optic spectrophotometer.

[0078] Step 4—Flush cassette: flush between samples with distilled water.

[0079] Step 5—Storage: after analyzing samples, do a final flush of cassette with distilled water, then remove the pump tubing from water and pump air through the cassette. Store dry.

[0080] Reagents:

[0081] Phenol/nitroprusside solution: combine phenol solution and sodium nitroprusside solution in 1:1 ratio. Dilute 10-fold using distilled water. Prepare fresh daily.

[0082] Phenol solution: mix 11.1 mL liquefied phenol (>=89%) with 95% v/v ethyl alcohol to a final volume of 100 mL. Prepare weekly.

[0083] Sodium nitroprusside solution: dissolve 0.5 g sodium nitroprusside in 100 mL deionized water. Store in amber bottle for up to 1 month.

[0084] Oxidizing Solution: combine alkaline citrate solution with sodium hypochlorite (bleach) at 4:1 ratio. Dilute 10-fold using distilled water. Prepare fresh daily.

[0085] Alkaline Citrate Solution: dissolve 100 g trisodium citrate and 5 g sodium hydroxide in deionized water. Dilute to 500 mL.

[0086] Comparison with APHA (1997) Standard Method 4500-NH.sub.3 F—phenate method:

[0087] Ammonium was analyzed using a cassette as described above, and also using APHA method 4500-NH.sub.3 F. For each method, a standard curve was generated using working standards containing 0, 5, 10, 15, 20, and 25 μM NH.sub.4.sup.+. Samples were collected from 5 different water sources. For quality control, each sample was analyzed with and without an ammonium spike (10 μM) and a known standard (10 μM) was analyzed after every 5 samples. The detection limit for each method was calculated as 3 times the minimum ammonium concentration necessary to generate a signal above background (0 μM ammonium). The standard method had lower limits of detection (2.25 μM for cassette method, 1.1 μM for method 4500-NH.sub.3 F). Calculated concentrations in control samples were similar between methods (8.4 μM for cassette method, 9.6 μM for method 4500-NH.sub.3 F. Generally, the cassette yielded lower calculated sample concentrations than method 4500-NH.sub.3 F, however this can be improved with increased length of the serpentine channel to increase reaction time before the sample reaches the reading cell. The cassette method resulted in less waste generation than method 4500-NH.sub.3 F (˜2 mL per sample versus ˜5.5 mL per sample).

EXAMPLE 3

Phosphate Cassette

[0088] Similar to Example 1, a multichannel peristaltic pump is used to introduce a water sample and a color reagent through 10 and 20 (FIG. 3), respectively. The color reagent contains ammonium molybdate and potassium antimonyl tartrate, which react with ortho-phosphate in the water sample, producing phosphomolybdic acid. Ascorbic acid, another constituent of the colour reagent, reduces the phosphomolybdic acid to molybdenum blue. The combined sample then enters reading cell 70 with a 4 cm path-length. Light absorbance at 880 nm is measured using a fiber optic light source and spectrophotometer. The absorption of light at this wavelength is proportional to the amount of dye, which is, in turn, proportional to the original concentration of ortho-phosphate.

[0089] Peristaltic pump speed can be controlled to increase or decrease reaction time, depending on sensitivity required versus speed of analysis. The minimum sample volume required for analysis is ˜1.3 mL, and the minimum color reagent volume required is ˜1.3 mL, creating a total waste per sample of <2.5 mL.

[0090] Procedure:

[0091] Step 1—Connect tubing: connect peristaltic tubing to ports (2.79 mm i.d. to ports 10 and 20).

[0092] Step 2—Run samples: pump water sample into port 10, color reagent into port 20.

[0093] Step 3—Record absorbance: once waste begins to exit through outlet port, stop pump and allow 5-10 minutes for colour development and then record absorbance from fiber optic spectrophotometer.

[0094] Step 4—Flush cassette: flush between samples with distilled water.

[0095] Step 5—Storage: after analyzing samples, do a final flush of cassette with distilled water, then remove the pump tubing from water and pump air through the cassette. Store dry.

[0096] Reagents:

[0097] Color Reagent: Combine 10 mL of reagent: 5 mL 5N H.sub.2SO.sub.4, 0.5 mL potassium antimonyl tartrate solution, 1.5 mL NH.sub.4 molybdate solution, 3 mL ascorbic acid solution. Dilute to 100 mL with distilled water.

[0098] Potassium Antimonyl: dissolve 1.3715 g K(SbO) C.sub.4H.sub.4O.sub.6×4H.sub.2O into 400 mL distilled water. Dilute to 500 mL with distilled water.

[0099] Ammonium Molybdate Solution: dissolve 20 g (NH.sub.4)6M.sub.O7O.sub.24 into 400 mL distilled water. Dilute to 500 mL with distilled water.

[0100] Ascorbic Acid: dissolve 1.76 g ascorbic acid into 100 mL distilled water.

[0101] Comparison with APHA (1997) Standard Method 4500-P E—molybdate/ascorbic acid method: Phosphate was analyzed using a cassette as described above, and also using APHA method 4500-P E. For each method, a standard curve was generated using working standards containing 0, 5, 10, 15, 20, and 25 μM PO.sub.4.sup.3−. Samples were collected from 5 different water sources. For quality control, each sample was analyzed with and without a phosphate spike (10 μM) and a known standard (10 μM) was analyzed after every 5 samples. The detection limit for each method was calculated as 3 times the minimum phosphate concentration necessary to generate a signal above background (0 μM phosphate). The cassette method had a lower limit of detection (<0.1 μM for cassette method, 0.3 μM for method 4500-P E). Percent recoveries in spiked samples were comparable between methods (average recovery 111% for cassette method, 106% for method 4500-P E). Generally, the cassette yielded slightly higher calculated sample concentrations than method 4500-P E. The cassette method resulted in less waste generation than method 4500-P E (˜2 mL per sample versus ˜5.5 mL per sample).

[0102] As many changes can be made to the preferred embodiment of the invention without departing from the scope thereof; it is intended that all matter contained herein be considered illustrative of the invention and not in a limiting sense. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.